In recent years, portable electronic devices, such as notebook personal computers, have been widely used. This type of portable electronic device is often operated by using a built-in battery, or the like, and it is not always connected to an external power source unlike stationary electronic devices. Therefore, portable electronic devices are connected to an external power source, such as an alternating-current (AC) adapter, to receive the electric power that is supplied from outside.
The above-described portable electronic device is sometimes configured to use an internal comparator so as to determine whether an AC adapter is connected. In the case of the configuration for determining whether an AC adapter is connected or not by using a comparator, it is preferable to prevent false detection as to whether an AC adapter is connected or not due to the current flowing into the comparator from other than the AC adapter. Therefore, a diode is often provided as a rectifying device of the power source line at an area after bifurcation of the route for supplying the current to the comparator in the supply route from the AC adapter. As the diode flows large current, the Schottky barrier diode is often used, which has less voltage drop in the forward direction.
Furthermore, for some of the above-described portable electronic devices, there are expansion units for the purpose of functionality expansion, or the like. Some of the expansion units are configured to receive the electric power from an external unit. Therefore, if a portable electronic device is connected to the expansion unit that receives the electric power that is supplied from an external unit, it may use the expansion unit as an external power source. In this case, the electric-power supply route from the expansion unit is often different from the electric-power supply route from the AC adapter.
As described above, some of the portable electronic devices have, as the route for supplying the electric power from an external unit, a route other than the route that uses an AC adapter as the external power source. In the case of portable electronic devices that have the electric-power supply route from an external power source other than an AC adapter, if the current, supplied from a different external power source, flows into the comparator that determines whether the AC adapter is connected or not, there is a possibility that the AC adapter is improperly detected.
Here, the Schottky barrier diode prevents the flow of the current into the comparator from other than the AC adapter and, due to its characteristics, it has large leak current when the voltage is applied in the backward direction. Furthermore, it is known that, if the temperature increases, the leak current of the Schottky barrier diode increases by about ten to hundred times.
Therefore, if a different external power source is connected without attaching an AC adapter, the voltage is applied to the Schottky barrier diode from the different external power source, and the leak current occurs. Particularly, if the temperature is high, more leak currents occur, and a high voltage is applied to the comparator, which may result in false detection of the AC adapter.
Here, the level of the leak voltage, which occurs due to the leak current, is represented by using the value that is obtained by multiplying the leak current by the impedance of the anode side of the Schottky barrier diode relative to the ground (GND). Specifically, if the impedance of the anode side of the Schottky barrier diode relative to the GND is decreased, the leak voltage may be reduced. Thus, it is considered that, in order to decrease the impedance, the resistance between the anode of the Schottky barrier diode and the ground is reduced.
According to a conventional technology, as the measures against the above-described leak current, in the configuration where an AC adapter and a battery are provided to the system side by being OR-connected by diodes, a battery-connection detection terminal is separately provided so as to prevent false detection of the voltage due to the leak current from the AC adapter. Furthermore, according to a conventional technology, in the configuration where the voltage sources in dual system are OR-connected, the same type of diode is used as the sink source for the leak current so as to prevent false voltage detection due to the leak current of a diode in the high-voltage environment. Furthermore, there is a conventional technology where, in the circuit where an AC adapter and a battery are OR-connected by diodes, the sink current in the circuit, which consumes the leak current of the diode, is changed in accordance with the temperature.
Japanese Laid-open Patent Publication No. 08-140289
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However, in order to set a low resistance, if the leak current at the high temperature is about 1 mA to 10 mA, it is possible to use the resistor with a low resistance value, e.g., 10Ω to 1 k Ω, so as to ensure its operation at the high temperature. In a case where such a resistor is used, if an operation is performed with the AC adapter of, for example, 20 V, an high standby electricity of 4 W to 400 mW occurs. This high standby electricity does not meet the standby electricity regulation, such as Energy related Product (Erp) regulation.
For example, the ErP regulation defines that the standby electricity at the primary side is equal to or less than 500 mW if an AC adapter is connected to the computer whose power source is mechanically turned off while there are no charges. Here, the efficiency of an AC adapter during low loads is about 50%. Therefore, the loss at the computer side is kept at equal to or less than 250 mW. Here, if the threshold for detecting an adapter is 15 V and if the leak current of the used diode at the high temperature reaches 4 mA, the impedance is kept at 15/4=3.75 kΩ). At least the loss due to this resistance causes the loss of 20×20/3.75=107 mW, and almost 40% of the standby electricity margin is lost. Therefore, if the resistor with a low resistance value is used, it is difficult to satisfy the condition for the loss at the computer side, and it is difficult to meet the standby electricity regulation.
Furthermore, in the case of application of the conventional technology for separately providing a detection terminal, new designing and manufacturing for the detection terminal are conducted, which results in an increase in costs and complexity of the manufacturing process. Furthermore, in the case of the conventional technology that uses the diode as the sink source for the leak current, there are few products that include multiple diodes, and new designing and manufacturing are needed, which results in an increase in costs. Furthermore, according to this conventional technology, the size of the substrate is bigger, and there is a possibility that it is difficult to install it in the computer. Furthermore, in the case of the conventional technology of changing the sink current in the circuit, which consumes the leak current of the diode, in accordance with the temperature, in order to switch the consumption circuit, the voltage after the OR connection is detected and is compared with a threshold. This method is unusable unless the condition is such that there is a difference in the voltage between the AC adapter and a different power source, and it is difficult to use it for multiple external power sources. Furthermore, according to this conventional technology, the sink current is not stepwise with regard to the temperature; therefore, when the leak current increases in an exponential fashion, if the consumption circuit is selected in accordance with the high temperature, there is a possibility that the sink current at the normal temperature becomes too high.